1. Field of the Invention
The present invention relates to a punching apparatus for punching holes in a sheet and, in particular, to stop position control of a cam member operated by a motor.
2. Description of the Related Art
Conventionally, a punching apparatus is incorporated in a sheet processing apparatus for punching holes in a sheet discharged from an image forming apparatus. In a punching apparatus discussed in U.S. Pat. No. 7,073,706, a motor moves a cam member for moving a punch up and down, and the punch is inserted into a die hole, to punch holes in a sheet.
Regions where the cam member is positioned include a punching region where the cam member is positioned when the punch is inserted into the die hole and a stop region where the cam member is positioned when the punch is not inserted into the die hole. When the cam member is in the punching region, the punch blocks a paper path, so that the sheet cannot be conveyed. In order to convey the subsequent sheet to the punching apparatus, therefore, the cam member must be moved and reliably stopped in the stop region after holes are punched in the sheet.
On the other hand, as the motor for driving the cam member, a DC motor is used to cope with to an unexpected large torque generated when the holes are punched in the sheet. However, the DC motor may not be immediately stopped because of the inertia even when a brake is applied and may overrun a target stop position by a predetermined amount. The faster the rotational speed of the motor at the time of punching, the stronger an inertial force, and the larger an amount of overrun becomes. In the conventional punching apparatus, therefore, the cam member can be stopped in the stop region by applying the brake to the motor before entering the stop region.
On the other hand, an amount of movement of the cam member by the overrun varies for each punching apparatus depending on a variation in a braking force of the motor and a variation in a frictional force of a driving structure. When the brake is applied to the motor before the cam member enters the stop region, the cam member may not be able to reach the stop region if the amount of overrun is too small. On the other hand, the cam member may be stopped in the punching region without being stopped in the stop region if the amount of overrun is too large.
In order to prevent such inconvenience, in U.S. Pat. No. 7,172,185, a sensor for measuring an amount of rotation of a motor is used to measure an amount of overrun after an elapse of a predetermined period of time since the brake was applied to the motor, to confirm overrun. Braking timing of the motor is controlled based on the measured amount of overrun so that a stop position of a punching edge is placed within a predetermined range.
In the conventional method which measures an amount of rotation in a period of time elapsed since the brake was applied to a motor until the motor is stopped using the sensor, when a position where the motor is stopped is in the vicinity of a boundary of a detection range (a detection edge) of the sensor, an amount of overrun may, in some cases, be erroneously detected by the vibration during the stop of the motor. More specifically, a detection member that moves along with a cam member comes and goes on the detection edge of the sensor so that an output of the sensor changes. Therefore, it may be erroneously detected that the cam member moves, though it is already stopped. When the amount of overrun is erroneously detected, the braking timing cannot be satisfactorily controlled. Thus, the motor is stopped with the punching edge projecting onto a paper path, resulting in jams. When a plurality of sensors is used such that the rotational direction of the motor can be detected to prevent the erroneous detection, by adding the sensors, the punching apparatus increases in cost and size and a detection circuit becomes complicated. When the braking timing is set to a fixed value, a high-cost motor that hardly varies in an amount of overrun must be used.
It is also possible that when the brake is applied to the motor, a movement region of the cam member is configured to be mechanically limited such that the cam member does not overrun. However, a shock occurring at the time when the cam member has reached a limit region results in the reduction in the life of the motor.